1. Field of the Invention
This invention relates to a rotational angle detector used for, for example, detecting a rotational angle of the steering wheel of a vehicle.
2. Description of the Related Art
The headlight control systems of a vehicle, which have been developed for practical use, detect a rotational angle of the steering wheel on the basis of a relative rotational angle signal sent from a relative rotational angle sensor to automatically control the change in the beam direction of the headlights in the right-left direction in accordance with the rotational angle thus detected.
The Electronic Stability Control Systems of a vehicle, which have been developed for practical use, detect a rotational angle of the steering wheel based on an absolute rotational angle signal from an absolute rotational angle sensor to prevent skidding based on the rotational angle thus detected and the like.
For developing a new-type vehicle equipped with both the headlight control system and the electronic stability control system, a conceivable procedure is the use of the two systems without any change from the viewpoint of the period of development.
In this case, it is necessary to equip a vehicle with a relative rotational angle sensor for the headlight control system and an absolute rotational angle sensor for the Electron Stability Control system. This results in an increase in the size of the sensor, leading to an increase in cost.
Accordingly, a desirable rotational angle detector has designed to obtain both the absolute rotational angle and the relative rotational angle.
Examples of such a rotational angle detector capable of detecting an absolute rotational angle and a relative rotational angle include the rotational angle detector disclosed in Japanese Examined Patent Publication H3-61886.
As shown in FIG. 7, in this rotational angle detector, a disk 30 having first slits 31 provided at regular angular intervals along its peripheral edge is attached to a steering shaft 1, and first and second optical sensors 34, 35 each comprising a combination of a light-emitting diode and a phototransistor are placed in connection with the first slits 31, to provide an A-phase pulse and a B-phase pulse in accordance with the passage of the first slits in association with the rotation of the steering shaft 1.
The first and second optical sensors 34, 35 are arranged such that the phases of the A-phase and B-phase output pulses are separated from each other by one-quarter period.
In addition, a second slit 32 is provided in the disk 30 for detection of the neutral position. A third optical sensor 36 outputting a Z-phase pulse is disposed for detecting the second slit 32 when the steering shaft 1 is placed in the neutral position.
According to Japanese Examined Patent Publication H3-61886, the use of either an A-phase output pulse or a B-phase output pulse makes it possible to provide a rotation change from any selected position of starting the rotation and also to find the rotating direction from the order of change in the correlation between A phase and B phase. In short, a relative rotational angle is provided.
FIG. 8 shows a pulse pattern of such a relative rotational angle signal.
Also, the number of rotations of the disk 30 can be found by counting the number of times that the third optical sensor 36 detects the second slit 32 from the Z phase. For this reason, based on this count value and a relative rotational angle obtained from A phase and B phase, it is possible to obtain the rotation angle as an absolute rotational angle from the neutral position even when the rotation goes through more than 360 degrees.